Thick film conductor compositions for use in membrane switch applications

ABSTRACT

The present invention relates to a thick film conductor composition comprising: (a) electrically conductive silver powder; (b) PVDF/HFP polymer resin, copolymers of a PVDF/HFP polymer resin, and mixtures thereof; dissolved in (c) organic solvent, with the provisos that the PVDF/HFP resin has a melt viscosity of 0.2-0.7 kPoise and a DSC melt temperature in the range of 85-98° C.

FIELD OF THE INVENTION

The invention is directed to improved thick film conductor compositionsand in particular to such compositions which are resistant to silvermigration in membrane touch switch applications.

BACKGROUND OF THE INVENTION

Polymeric thick film inks consist of particles containing conductivematerials dispersed in an organic vehicle or medium containing volatilesolvent and a polymeric resin. After screen-printing, the composition isdried typically by heating at temperatures of up to 150° C. where theorganic solvent is dried-off.

The conductive material is responsible for imparting to the thick filmmaterial the desired level of resistivity. This is normally limited toless than 0.02 Ohm/sq/mil. The conductive particles typically consist ofsilver metal for high conductivity and good resistance to oxidation andcan be found in flake or non-flake morphologies.

After drying, the polymeric resin's primary function is to bind togetherthe conductive particles to form an electrically conductive circuitpattern. Additionally, the binder system is required to impart thenecessary adhesion to the desired substrate. In the case of flexiblesubstrates which may or may not be surface treated, thermoplastic bindersystems are typically employed. In general, these consist of polyesters,acrylics, vinyl or polyurethane polymers and may be combined to obtainoptimum properties.

Furthermore, the resinous component is also responsible for providingthe conductor composition with the required surface hardness, resistanceto environmental changes and flexibility.

Historically, linear polyester, acrylic or vinyl copolymer-based resinshave been typically been used as the polymeric binder for generalpurpose membrane touch switch (MTS) conductor pastes. Other types ofresins, such as polyurethane resins have also been utilized in membranetouch switch applications.

U.S. Pat. No. 4,595,605 discloses conductive compositions that aresolderable and flexible and that can be bonded directly to substrates.These compositions are made from a combination of silver exclusively inthe form of flake and vinyl chloride/vinyl acetate copolymer.

U.S. Pat. No. 4,371,459 teaches a screen printable conductor compositionuseful in membrane touch switch compositions that is flexible comprising(a) a conductive phase containing silver and base metal powdersdispersed in a solution of (b) a multipolymer prepared bycopolymerization of vinyl acetate, vinyl chloride, and an ethylenicallyunsaturated dicarboxylic acid and a linear aromatic polyester resindissolved in (c) volatile nonhydrocarbon solvent.

U.S. Pat. No. 5,653,918 to Towlson discloses a highly flexible andmechanically robust screen-printable conductor composition containing(a) a conductive phase comprising Ag, Au, Cu, Ni, Pd, Pt, C or graphiteand mixtures thereof dispersed in a polymer solution containing (b) aterpolymer of polyvinyl acetate, vinyl chloride and a polar componentdissolved in a volatile solvent.

Although the above-listed patents cite improvements in adhesion of thecomposition to the desired substrate and durability, none of the listedpatents appear to improve overall lifetime of the touch switch andcontrol of silver migration. The present invention provides a superiorthick film conductive composition which increases MTS lifetime bydecreasing metal conductor migration.

SUMMARY OF THE INVENTION

The present invention relates to a thick film conductor compositioncomprising: (a) electrically conductive silver powder; (b) PVDF/HFPpolymer resin, copolymers of a PVDF/HFP polymer resin, and mixturesthereof; dissolved in (c) organic solvent, with the provisos that thePVDF/HFP resin has a melt viscosity of 0.2-0.7 kPoise and a DSC melttemperature in the range of 85-98° C.

The invention further relates to a method of forming a membrane touchswitch comprising: (a) preparing the composition as described above; (b)applying the composition of (a) onto a substrate; (c) drying thecomposition of (b) to form a circuit; and (d) applying a voltage acrossthe circuit of (c).

Additionally, the present invention relates to a membrane touch switchutilizing the composition described herein and above and a membranetouch switch formed by the method described herein.

DETAILED DESCRIPTION OF INVENTION

Generally, a thick film composition comprises a functional phase thatimparts appropriate electrically functional properties to thecomposition. The functional phase comprises electrically functionalpowders dispersed in an organic medium that acts as a carrier for thefunctional phase. Generally, the composition is fired to burn out theorganics and to impart the electrically functional properties. However,in the case of polymer thick film, the organics remain as an integralpart of the composition after drying. Prior to firing, a processingrequirement may include an optional heat treatment such as drying,curing, reflow, and others known to those skilled in the art of thickfilm technology. “Organics” comprise polymer or resin components of athick film composition.

The main components of the thick film conductor composition are aconductor powder dispersed in an organic medium, which is comprised ofpolymer resin and solvent. The components are discussed herein below.

A. Conductor Powder

The electrically functional powders in the present thick filmcomposition are Ag conductor powders and may comprise Ag metal powder,alloys of Ag metal powder, or mixtures thereof. The particle diameterand shape of the metal powder is not particularly important as long asit is appropriate to the application method. Virtually any shape silverpowder, including spherical particles, and flake (rods, cones, plates)may be used in practicing the invention.

The particle size distribution of the metal particles is not itselfcritical with respect to the effectiveness of the invention. However, asa practical matter, it is preferred that the particles size be in therange of 1 to 100 microns and preferably 2-10 microns.

In addition, it is preferred that the surface area/weight ratio of thesilver particles are in the range of 0.1-1.0 m²/g.

Furthermore, it is known that small amounts of other metals may be addedto silver conductor compositions to improve the properties of theconductor. Some examples of such metals include: gold, silver, copper,nickel, aluminum, platinum, palladium, molybdenum, tungsten, tantalum,tin, indium, lanthanum, gadolinium, boron, ruthenium, cobalt, titanium,yttrium, europium, gallium, sulfur, zinc, silicon, magnesium, barium,cerium, strontium, lead, antimony, conductive carbon, and combinationsthereof and others common in the art of thick film compositions. Forexample, palladium is frequently added to silver conductor compositions.The additional metal(s) may comprise up to about 1.0 percent by weightof the total composition.

B. Organic Medium

The powders are typically mixed with an organic medium (vehicle) bymechanical mixing to form a pastelike composition, called “pastes”,having suitable consistency and rheology for printing. A wide variety ofinert liquids can be used as organic medium. The organic medium must beone in which the solids are dispersible with an adequate degree ofstability. The rheological properties of the medium must be such thatthey lend good application properties to the composition. Suchproperties include: dispersion of solids with an adequate degree ofstability, good application of composition, appropriate viscosity,thixotropy, appropriate wettability of the substrate and the solids, agood drying rate, and a dried film strength sufficient to withstandrough handling. The organic medium is not conventional in the art, istypically a solution of polymer in solvent(s), and lends uniqueproperties to the composition.

The polymer resin of the present invention is particularly important.The resin used in the present invention is a member of thefluorocarbon-resin family, polyvinylidene fluoride/hexafluoropropylene(PVDF/HFP) and copolymers thereof, made by polymerizing1,1-difluoroethylene, H₂C═CF₂, a colorless gas. The resin is thermallystable to high temperatures, is stronger and more abrasion-resistantthan other fluoroplastics, and is easier to process on conventionalthermoplastics equipment. Additionally, the polymer resin of the presentinvention is characterized by the following physical characteristics:(1) melt viscosity of 0.2-0.7 kPoise; (2) DSC melt temperature of 85-98°C.; and (3) mole % of hexafluoropropylene (HFP) in total resincomposition of about 12-16 mole %.

The most widely used solvents found in thick film compositions are ethylacetate and terpenes such as alpha- or beta-terpineol or mixturesthereof with other solvents such as kerosene, dibutylphthalate, butylcarbitol, butyl carbitol acetate, hexylene glycol and high boilingalcohols and alcohol esters. In addition, volatile liquids for promotingrapid hardening after application on the substrate can be included inthe vehicle. In many embodiments of the present invention, solvents suchas glycol ethers, ketones, esters and other solvents of like boilingpoints (in the range of 180° C. to 250° C.), and mixtures thereof may beused. The preferred mediums are based on glycol ethers and β-terpineol.Various combinations of these and other solvents are formulated toobtain the viscosity and volatility requirements desired.

The solids are mixed with the organic medium by mechanical mixing usinga planetary mixer, then dispersed on a three roll mill to form apaste-like composition having suitable consistency and rheology forscreen printing. The latter is printed as a “thick film” on substratesin the conventional manner as known to those in the art of thick filmtechnology.

The ratio of organic medium in the thick film composition to theinorganic solids in the dispersion is dependent on the method ofapplying the paste and the kind of organic medium used. Normally toachieve good coverage, the dispersions will contain complementarily50-91% wt. inorganic solids and 50-9% wt. vehicle, as described above.The compositions of the present invention may, of course, be modified bythe addition of other materials, which do not affect its beneficialcharacteristics. Such formulations are well within the state of the art.

The pastes are conveniently prepared on a three-roll mill. The viscosityof the pastes is typically within the following ranges when measured ona Brookfield HBT viscometer at low, moderate and high shear rates: ShearRate (RPM) Viscosity (Pa * s) 0.5  50-2500 150-1000 Preferred 300-750 Most Preferred 10 20-200 50-125 Preferred 60-100 Most Preferred 100 5-7512.5-60   Preferred 25-50  Most PreferredApplication of Composition

The composition of the present invention may be used in the formation ofa membrane touch switch. The membrane touch switch may be formed by amethod comprising: (a) preparing the composition of the presentinvention; (b) applying the composition of a) onto a substrate; (c)drying the composition of b) to form a circuit; and (d) applying avoltage across the circuit of (c).

Additionally, the present invention relates to a membrane touch switchutilizing the composition described herein and above and a membranetouch switch formed by the method described herein.

EXAMPLES

This invention will now be described in further detail with practicaland comparative examples (Examples 1-2).

Example 1

(Representing Present Invention)

25.0 grams (g) of an experimental VDF/HFP co-polymer (provided by SolvaySolexis and described above in the Detailed Description) was dissolvedin 75.0 g. of diethylene glycol ethyl ether acetate and stirred at 95 Cuntil all resin dissolved, and then cooled to room temp. 25.0 g. of thisorganic vehicle was then mixed with 75.0 g. of flake silver (averageparticle size 5 microns) for 30 minutes. After this time, the mixturewas subjected to several passes on the three-roll mill at approximately150 PSI. Following this, approximately 5.0 g. of the above solvent wasadded to reduce the viscosity of the resulting paste, which will bereferred to as silver paste “A”. Silver migration test parts were thenscreen printed using a 280 Stainless Steel mesh screen containing a 0.5mil emulsion, and were dried at 130° C. for 10 minutes in a box oven.The pattern printed was that of approximately 2 inch long silver tracesseparated by approximately 40 mils, in an attempt to simulate a typicalmembrane touch switch (MTS) circuit.

The substrate used was MYLAR, a common one in the MTS industry. Thevoltage employed was 10 V DC and these parts were exposed to approx. 85degrees C./85% relative humidity until the point of failure wasachieved. Point of failure is defined as a “short circuit” between twoadjacent traces caused by silver migration. The longer the number ofhours to failure, the better the silver conductor is in terms of silvermigration-resistance.

Example 2

(Comparative Experiment with Prior Art)

Similar procedures were followed as above except that DuPont silverconductor composition 5007 (Available from the E. I. du Pont de Nemoursand Company located in Wilmington, Del.) which contains 63.0 wt. % flakesilver and a polyester resin dissolved in the same solvent as above] wasused in place of the silver paste “A” noted above. This will be referredto as silver paste “B”.

Results of Silver Migration Testing Measured as Hours to Failure at 85°C./85% Relative Humidity 10V DC

Silver Paste A 628 hours (mean value) Silver Paste B  90 hours (meanvalue)

As can be seen above, there is a significant difference between thestandard MTS conductor (5007) and that of silver paste “A” (Compositionof the present invention), the latter being much more resistant tosilver migration. Testing at 85° C./85% relative humidity is oftenreferred to as accelerated aging as it simulates years of lifetime in amuch shorter period of time. Using established formulas, one canapproximate the lifetimes of circuits made with the above compositions.Employing those formulas known to those skilled in the art, oneestimates the lifetime of circuits made with silver paste “A” as 30.2years while that of silver paste “B” as 4.2 years.

1. A thick film conductor composition comprising: a) electricallyconductive silver powder; b) PVDF/HFP polymer resin, copolymers of aPVDF/HFP polymer resin, and mixtures thereof; dissolved in c) organicsolvent. with the provisos that the PVDF/HFP resin has a melt viscosityof 0.2-0.7 kPoise and a DSC melt temperature in the range of 85-98° C.2. The composition of claim 1 wherein the PVDF/HFP resin contains about12-16 mole % of hexafluoropropylene (HFP) in the total resincomposition.
 3. The composition of claim 1 wherein the boiling point ofthe organic solvent is in the range of 180° C. to 250° C.
 4. Thecomposition of claim 1 wherein the organic solvent is selected from thegroup comprising glycol ethers, ketones, esters, and mixtures thereof.5. The use of the composition of claim 1 in membrane touch switchapplications.
 6. A method of forming a membrane touch switch comprising:a) preparing the composition of claim 1; b) applying the composition ofa) onto a substrate; c) drying the composition of b) to form a circuit;and d) applying a voltage across the circuit of c).
 7. A membrane touchswitch utilizing the composition of claim
 1. 8. A membrane touch switchformed by the method of claim 6.